In a vehicle equipped with an engine, as a transmission that appropriately transmits torque and the speed of revolution generated by the engine to drive wheels according to the running state of the vehicle, an automatic transmission is known that automatically optimally sets a gear ratio between the engine and the drive wheels.
Examples of an automatic transmission mounted in a vehicle include a planetary gear transmission that sets a gear using frictionally engaging elements such as a clutch and a brake and a planetary gear apparatus, and a belt-driven stepless transmission (CVT: Continuously Variable Transmission) that steplessly adjusts the gear ratio.
In a vehicle in which a planetary gear-type automatic transmission is mounted, a gearshift map that has gearshift lines (gear switching lines) for obtaining an optimal gear according to the vehicle speed and the accelerator opening degree (or throttle opening degree) is stored in an ECU (Electronic Control Unit) or the like, a target gear is calculated with reference to the gearshift map based on the vehicle speed and the accelerator opening degree, and based on that target gear, a gear (selected gear) is automatically set by engaging or disengaging a clutch, a brake, a one-way clutch, and the like, which are frictionally engaging elements, in a predetermined state.
In the configuration of a belt-driven stepless transmission, a belt is wrapped around a primary pulley (input side pulley) and a secondary pulley (output side pulley) that are provided with a pulley groove (V groove), and by reducing the groove width of one pulley while increasing the groove width of the other pulley, the contact radius (effective diameter) of the belt to each of the pulleys is continuously changed to steplessly set a gear ratio.
In a vehicle equipped with such an automatic transmission, a torque converter is disposed in a power transmission path from the engine to the automatic transmission. The torque converter, for example, is provided with a pump impeller connected to an engine output shaft (crankshaft), a turbine runner connected to an input shaft of the automatic transmission, and a stator provided between the pump impeller and the turbine runner via a one-way clutch. The torque converter is a hydraulic transmission apparatus in which the pump impeller rotates according to rotation of the engine output shaft, and the turbine runner is rotationally driven by operating oil discharged from the pump impeller, thus transmitting engine output torque to the input shaft of the automatic transmission.
The torque converter is provided with a lockup clutch that directly connects its input side (pump side) and output side (turbine side), and lockup engagement control is executed to bring the lockup clutch into engagement so as to directly connect the input side and the output side of the torque converter. Lockup slippage control (flex lockup control) is also executed to bring the lockup clutch into half-engagement that is intermediate between engagement and disengagement (see, for example, Patent Documents 1 and 2). By executing such lockup engagement control and lockup slippage control, an improvement in fuel economy can be achieved.
The lockup engagement control as used herein (hereinafter, it may be simply referred to as “engagement control”) uses a map in which a lockup engagement region (lockup-on region) and a disengagement region (lockup-off region) are set using vehicle running state parameters such as vehicle speed and throttle opening degree and brings the lockup clutch into engagement or disengagement based on the actual vehicle running state (for example, vehicle speed and throttle opening degree) in reference to the map.
Also, the lockup slippage control (hereinafter, it may be simply referred to as “slippage control”) similarly uses a map in which a lockup slippage region and a disengagement region are set using vehicle running state parameters such as vehicle speed and throttle position and brings the lockup clutch into slipping (half-engagement) or disengagement based on the actual vehicle running state (for example, a vehicle speed and a throttle position) in reference to the map.